Water Dispersible Clay Research Articles

Factors Contributing to the Clay Dispersion and Aggregate Stability of Thai Oxisols

Aggregate stability tends to give a better availability of soil water. Clay dispersion makes the soil dense, difficult for root penetration. The better understanding of these two soil properties is necessary to evaluate the suitability of soils for plant production. We collected soil samples from 10 sites from the southeast coast and northeast plateau of Thailand, at upper (0 - 5 cm depth) and lower (5 - 20 cm depth) levels for under udic and ustic Oxisols soil samples. The objective of this study was to investigate the contribution of major aggregate binding agents such as soil organic carbon (SOC), Fe and Al oxides, clay minerals and kaolin crystal size to the stability of soil aggregates. Soils were acidic (pH < 6.5), low to medium cation exchange capacity (3.34 - 14.40 cmol kg -1 ), SOC ranged from 2.08 - 4.56 % for udic and 1.11 - 1.56 % for ustic soils. The udic soils showed higher contents of crystalline, non-crystalline, and organic forms of Fe and Al than those for ustic soils. The mean coherently scatterings domain (CSD) values for kaolin crystal size were 115 and 152 nm for udic and ustic soils, respectively. The mean water dispersible clay (WDC), water dispersible silt (WDSi), and dispersion ratio (DR) were 92, 43 g kg -1 , and 0.17 for udic soils, and 218, 45 g kg -1 , and 0.30 for ustic soils, respectively. Mean weight diameters (MWD) were 1.01 and 0.62 mm for udic and ustic soils, respectively. The clay flocculation index (CFI) and aggregated silt and clay (ASC) were 0.83 and 650 g kg -1 for udic soils and 0.66 and 619 g kg -1 for ustic soils. The SOC, non-crystalline, organic form of Fe and Al oxides, and crystalline Al oxide showed positive correlation to MWD and CFI. A negative correlation between crystal size of kaolin and aggregate stability was found. Graphical abstract

Soil resistance and resilience to mechanical stresses for three differently managed sandy loam soils

Abstract To improve our understanding of how clay-organic carbon dynamics affect soil aggregate strength and physical resilience, we selected three nearby soils (MFC, Mixed Forage Cropping; MCC, Mixed Cash Cropping; CCC, Cereal Cash Cropping) with identical clay content and increasing contents of organic carbon (CCC k a ), void ratio ( e ) and air-filled porosity ( e ) as functional indicators and to characterise aggregate stability, strength and friability. Aggregate tensile strength measurements were done at − 1000 hPa and in the air dried state. Soil cores were subjected to uniaxial confined compression (200 kPa) followed by a period of natural recovery and wet–dry or freeze–thaw cycles. The MFC soil displayed greater tensile strength, specific rupture energy and friability at both soil-water potentials than the MCC and CCC soils possibly due to higher biotic binding of soil particles by the greater organic carbon content. The water dispersible clay was negatively correlated with the level of clay saturation by organic carbon. The resistance of the soils to compaction, quantified by both the compression index and a proposed functional index, was significantly greater for the MFC soil compared to the other two soils. The change in compression index with initial void ratio was significantly less for the MFC than the other soils. Plastic reorganisation of the soil particles immediately after compaction, prior to removal of the load, was greatest for the high organic soil. Physical resilience after natural recovery followed the organic carbon gradient (MFC > MCC > CCC). After wet–dry cycles, the MFC soil showed a significantly greater resilience for all three indicators ( k a , e and e ). Resilience ( k a , and e ) after freeze–thaw cycles was significantly lower for the CCC soil than the MFC and MCC soils. A significant positive correlation between resistance and resilience was observed for k a and e but not for e . Resilience to compaction was found to be independent of the soil properties monitored.

CLAY DISPERSION IN TYPICAL SOILS OF NORTH CAMEROON AS A FUNCTION OF PH AND ELECTROLYTE CONCENTRATION

ABSTRACTKnowledge of clay dispersion behaviour [which is highly influenced by ion concentration in the aqueous phase and by related surface charge (SC) of colloids] is important for rating soil erosion risk (by water). It can also be useful for improving soil management systems. Clay fractions separated from samples of the A‐horizon of a Vertisol, Ultisol and Oxisol were collected, representing typical soils of North Cameroon. These soils were very different in physicochemical and mineral parameters. The effect of pH and the multivalent ions Ca2+, SO42− and PO43− on SC and dispersion characteristics were determined. The water dispersible clay was found to be higher in the Vertisol and Ultisol than in the Oxisol, indicating that the &lt;2 µm fractions from the Vertisol and Ultisol are more dispersible than that from the Oxisol. The clay dispersion ratio together with the dispersion ratio were found to be in good agreement with water dispersible clay and are negatively correlated with the amount of organic matter and dithionite citrate bicarbonate soluble Fe and Al. Generally, SC of the &lt;2 µm fraction was found to be negative when the pH was in the region of 3 to 9; thus the absolute value is highly pH‐dependent. At pH 6 and 8, CaCl2, Na2SO4 and Na2HPO4 additions had antagonistic effects on SC: Ca2+ increased SC, whereas SO42− and PO43−decreased SC indicating the adsorption of positively as well as negatively charged multivalent ions by soil colloids. Along with the increase of SC, there was a fall in repulsive forces and formation of Ca‐bridges, the addition of Ca2+ induced flocculation more rapidly than SO42− and PO43− amendments. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Sediment source identification in a semiarid watershed at soil mapping unit scales

Selective erosion and transport of silt and clay particles from watershed soil surfaces leads to enrichment of suspended sediments by size fractions that are the most effective scavengers of chemical pollutants. Thus, preferential transport of highly reactive size fractions represents a major problem relative to sediment/chemical transport in watersheds, and offsite water quality. The objective of this research was to develop an approach to identify sediment sources at a soil mapping unit scale for the purpose of designing site specific best management practices which affect greater reductions in runoff and erosion losses. Surface soil samples were collected along transects from each of the major 25 mapping units in six subwatersheds of the Walnut Gulch Experimental Watershed. Suspended sediments were collected from supercritical flumes at the mouth of each subwatershed. Laboratory analyses included basic soil/sediment physical and chemical properties, radioisotopes, and stable carbon isotopes, all by standard methods. Aggregation index (AI) values [100 · (1 − water dispersible clay / total clay)] were taken as an indicator of relative soil erodibility. Potential sediment yield index (PSYI) values were calculated by multiplying percent relative area for individual soil mapping units times (100 − AI). Particle size results indicated that suspended sediments were enriched in clay, relative to the watershed soils, by an average of 1.28. Clay enrichment ratios (ER) were significantly (P ≤ 0.01) and positively correlated with AI, an indication that these two parameters can be equated with erodibility and sediment yield. The PSYI values for the six subwatersheds ranged from 68.0 to 81.7. The stable carbon isotope data for the suspended sediments gave a C3 (shrubs) to C4 plant (grasses) ratio that ranged from 1.06 to 2.25, indicating greater erosion from the more highly erodible, shrub-dominated subwatersheds which also coincided with the highest PSYI values. Correlation coefficients determined individually for PSYI versus clay ER, C3/C4 plant ratios, and multivariate mixing model results were: 0.962 (P ≤ 0.01), 0.905 (P ≤ 0.01), and 0.816 (P ≤ 0.05), respectively. These statistically significant relationships support the accuracy of a potential sediment yield index approach for identifying suspended sediment sources at soil mapping unit scales.

Qualidade física e química do solo em áreas de exploração florestal no Mato Grossso

Com o objetivo de avaliar o impacto provocado nos solos sob pátios de estocagem de madeira, bem como a influência do formato dos pátios sobre o processo de degradação nesses locais, foram selecionados cinco pátios em uma área de exploração de madeira, onde predominava solo do tipo Latossolo Vermelho-Amarelo, localizada no Município de Nova Maringá, MT. Foram escolhidos quatro pátios de formato quadrado e um de formato retangular. Em cada pátio, estabeleceram-se três transectos, que adentraram a mata nativa, sendo feitas coletas de solo na profundidade de 0 a 20 cm, em cinco pontos de cada transecto. Essas amostras foram utilizadas para a determinação de textura, argila dispersa em água, umidade na capacidade de campo, estabilidade e distribuição de agregados, análises químicas de rotina, C-total, N-total e C mineralizável. Para a determinação da densidade do solo e densidade de partículas, as amostras foram retiradas nas profundidades de 0 a 5 cm e 15 a 20 cm do transecto central. Os resultados foram submetidos a testes de média (Tukey, P&lt;0,05). Testes de resistência à penetração em cinco pontos de cada pátio e três pontos na mata adjacente foram realizados na época da seca e na época chuvosa. Os resultados mostraram que houve forte degradação nas propriedades dos solos sob pátios de estocagem, observando-se expressivo aumento na densidade do solo e resistência à penetração, bem como diminuição da porosidade total, do DMP dos agregados e dos teores de nutrientes. A utilização de pátios com diferentes formas não minimizou os impactos ambientais.

Soil degradation in Central North Cameroon: Water-dispersible clay in relation to surface charge in Oxisol A and B horizons

Water dispersible clay (WDC) is a good indicator for the risk of soil erosion by water. In this study, samples from the A and B horizons of Oxisols have been collected along a toposequence developed on basalt in Central North Cameroon. The aim was to determine the WDC of these horizons and to investigate the relation with soil physical and chemical properties. The effects of pH and different electrolytes on surface charge and clay dispersion were also assessed. Data collected included particle size distribution, water dispersible clay, clay dispersion ratio (CDR), clay flocculation index (CFI), pH, organic carbon content, cation exchange capacity (CEC), exchangeable cations, oxalate and dithionite soluble Fe and Al, mineralogical composition of the clay fraction, specific surface area and surface charge (SC). Clay fractions of Oxisol A and B horizons are constituted mainly by kaolinite, associated with gibbsite and Fe oxides. Total clay content of A horizons varies from 210 to 430 g kg −1, and in B horizons from 366 to 405 g kg −1. It is negatively correlated to the CEC, which is due to relatively high amounts of organic matter in the A horizons, and also to dithionite and oxalate extractable Fe and Al. The WDC ranges from 103 to 212 g kg −1 and from 5 to 98 g kg −1 in Oxisol A and B horizons, respectively. The WDC correlated positively with CDR, and both were strongly affected by soluble Na +. It appears that A horizons of the upper and middle slope are more dispersible than B horizons. Under varying pH (from 9 to 3), clay fraction from A horizon displayed quite lower surface charge than that from B horizon, being in good agreement with the trend observed by WDC. This finding is assigned to the presence of distinct amounts of organic matter in the A horizon, which, due to its negative charge, increases repulsive forces between soil colloids. Complete flocculation is observed at pH 3 and 4, the pH at which the zero point of charge is reached for clay fractions from A and B horizons, respectively. The multivalent ions Ca 2+, SO 4 2− and PO 4 3− have antagonistic effects on the SC of clay fractions: Ca 2+ increased SC while SO 4 2− and PO 4 3− decreased SC, but complete flocculation within 24 h is observed when each of these electrolytes is added even at the lowest concentration of 0.01 mM L −1. Some measures are suggested to avoid the effects of soluble Na + on the dispersion of these soils.

Relações solo-paisagem em uma topossequência sobre substrato granítico em Santo Antônio do Matupi, Manicoré (AM)

Considerando as dimensões da região amazônica e a necessidade de estudos que estabeleçam conexões entre os solos e a expressão da paisagem, o objetivo deste trabalho foi estudar as relações solo-paisagem em uma topossequência sobre substrato granítico em Santo Antônio do Matupi, Manicoré, AM. Foi estabelecido um caminhamento de 3.000 m a partir do "espigão" da vertente, no seu sentido mais suave, com identificação dos segmentos da vertente com base na "quebra" do declive do terreno. Foram abertas trincheiras nos segmentos de vertente mapeados, e perfis foram caracterizados morfologicamente, coletando-se amostras de seus horizontes. Foram realizadas análises físicas (textura, ADA e GF, Ds e Dp, Pt e CHSS), químicas (pH em água e KCl, Ca, Mg, K, Na e Al trocáveis, P disponível, H + Al e C orgânico; SiO2, Al2O3 e Fe2O3 (método do H2SO4) e óxidos de Fe "livres" extraído com ditionito-citrato-bicarbonato e o Fe mal cristalizado extraído com oxalato de amônio) e mineralógicas (frações areia, silte e argila), usando difratometria de raios X. As variações do relevo favoreceram a presença de solos distróficos nos topos e solos eutróficos no sopé de transporte. As variáveis ΔpH, relação silte/argila, V % e relação Fed/Fet indicaram a presença de solos mais intemperizados nas áreas de topo da paisagem e solos com menor grau de evolução na encosta e no sopé de transporte. A composição mineralógica da fração argila mostrou-se relativamente homogênea, com predominância de caulinita e gibbsita e menor proporção de mica e quartzo.

Caracterização e classificação de terras pretas arqueológicas na Região do Médio Rio Madeira

Apesar da existência de muitas pesquisas sobre as Terras Pretas Arqueológicas, poucos são os trabalhos que têm procurado entender e esclarecer dúvidas sobre a gênese e o comportamento destes solos. Dessa maneira, o objetivo deste trabalho foi caracterizar e classificar terras pretas arqueológicas de quatro sítios na Região do Médio Rio Madeira. Nesses locais, foram abertas trincheiras e os perfis caracterizados morfologicamente e amostrados. Foram realizadas as seguintes análises físicas: textura, argila dispersa em água, densidades do solo e das partículas, porosidade total e condutividade hidráulica saturada. As análises químicas constaram de: pH em água e KCl, cátions trocáveis, Al trocável, P disponível, acidez titulável (H+Al) e C orgânico, Al2O3 e Fe2O3 e SiO2 extraídos pelo método do ataque sulfúrico, e teor de C das substâncias húmicas. Para determinar óxidos de Fe foram utilizados o ditionito-citrato-bicarbonato (Fed) e o oxalato ácido de amônio (Feo). As análises mineralógicas das frações areia, silte e argila foram feitas por difratometria de raios-X. Os perfis foram classificados no Sistema Brasileiro de Classificação de Solos como: Argissolo Vermelho Amarelo, Argissolo Acinzentado e Argissolo Amarelo. Os solos apresentaram horizonte A antrópico com características químicas e de profundidade semelhantes, sugerindo certa similaridade dos fatores antrópicos que promoveram sua formação. Os horizontes antrópicos de todos os perfis amostrados na região tinham caráter eutrófico e teores altos a muito altos de fósforo disponível, sempre maiores que os dos horizontes subjacentes. Sugere-se a inclusão do subgrupo antrópico no SiBCS.

Water-dispersible clay in soils treated with sewage sludge

A by-product of Wastewater Treatment Stations is sewage sludge. By treatment and processing, the sludge is made suitable for rational and environmentally safe use in agriculture. The aim of this study was to assess the influence of different doses of limed sewage sludge (50 %) on clay dispersion in soil samples with different textures (clayey and medium). The study was conducted with soil samples collected from native forest, on a Red Latosol (Brazilian classification: Latossolo Vermelho distroférrico) loamy soil in Londrina (PR) and a Red-Yellow Latosol (BC: Latossolo Vermelho-Amarelo distrófico) medium texture soil in Jaguapitã (PR). Pots were filled with 3 kg of air-dried fine earth and kept in greenhouse. The experiment was arranged in a randomized block design with six treatments: T1 control, and treatments with limed sewage sludge (50 %) as follows: T2 (3 t ha-1), T3 (6 t ha-1), T4 (12 t ha-1), T5 (24 t ha-1) and T6 (48 t ha-1) and five replications. The incubation time was 180 days. At the end of this period, the pots were opened and two sub-samples per treatment collected to determine pH-H2O, pH KCl (1 mol L-1), organic matter content, water-dispersible clay, ΔpH (pH KCl - pH-H2O) and estimated PZC (point of zero charge): PZC = 2 pH KCl - pH-H2O, as well as the mineralogy of the clay fraction, determined by X ray diffraction. The results showed no significant difference in the average values for water-dispersible clay between the control and the other treatments for the two soil samples studied and ΔpH was the variable that correlated best with water-dispersible clay in both soils.

Irrigation with domestic wastewater: a multivariate analysis of main soil changes

Irrigation with treated domestic sewage wastewater (TSE) is an agricultural practice to reduce water requirements of agroecossystems and the nutrient load impact on freshwaters, but adverse effects on soil chemical (salinization, sodification, etc.) and soil physical properties (alteration in soil porosity and hydraulic conductivity, etc.) have been reported. This study aimed to define some relationships among these changes in an Oxisol using multivariate analysis. Corn (Zea mays L.) and sunflower (Helianthus annuus L.) were grown for two years, irrigated with TSE. The following soil properties were determined: Ca2+; Mg2+; Na+; K+ and H + Al contents, cationic exchangeable capacity (CEC), sum of bases (SB), base saturation (V), texture (sand, silt and clay), macro-, micro-, and cryptoporosity (V MA, V MI and V CRI), water content at soil saturation (θS) and at field capacity (θFC), residual water content (θR), soil bulk density (d s), water dispersed clay (WDC) and saturated hydraulic conductivity (K SAT). Factor analysis revealed the following six principal factors: Fine Porosity (composed of Na+; K+; WDC, θR, θRFC, and V CRI); Large Porosity (θS, d s, V MA, Vs); Soil CEC (Ca2+; Mg2+; CEC, SB, V); Soil Acidity (H + Al); and Soil Texture (factors 5 and 6). A dual pore structure appears clearly to the factors 1 and 2, with an apparent relationship between fine porosity and the monovalent cations Na+ and K+. The irrigation (with potable sodic tap water or sewage wastewater) only had a significant effect on Fine Porosity and Large Porosity factors, while factors 3 and 4 (Soil CEC and Soil Acidity) were correlated with soil depth. The main conclusion was a shift in pore distribution (large to fine pores) during irrigation with TSE, which induces an increase of water storage and reduces the capacity of drainage of salts.

Estabilidade de agregados em argissolo sob diferentes usos, comparado com mata

A estrutura exerce grande importância no comportamento agrícola do solo, pela sua influência em propriedades do solo. Assim, objetivou-se avaliar a estabilidade de agregados e relacioná-la com diferentes frações orgânicas de um ARGISSOLO Vermelho-amarelo distrófico sob mata, seringueira, laranjeira e pastagem degradada. A amostragem foi realizada em trincheiras, de 0 a 10, 10 a 20 e 20 a 30 cm. Foram analisadas textura, argila dispersa em água, grau de floculação, densidade do solo, carbono orgânico total (COT), nitrogênio total (NT), ácidos fúlvicos (AF), húmicos (AH) e humina (HN), carbono da biomassa microbiana (C-Micr) e matéria orgânica leve (MOL). Foi determinada a estabilidade de agregados em água e calculados índices de estabilidade. Os resultados mostram diferenças de densidade do solo, seguindo a ordem mata &lt; seringueira &lt; laranjeira &lt; pastagem. A estabilidade de agregados não se altera entre os usos nas profundidades de 0 a 10 e 10 a 20 cm, enquanto de 20 a 30 cm nota-se a ordem mata &gt; seringueira &gt; laranjeira &gt; pastagem. No solo sob mata, seringueira e laranjeria a estabilidade de agregados está relacionada aos maiores teores de COT, NT,AF, AH, HN, C-Micr e MOL, enquanto na pastagem está relacionada principalmente a ciclos de umedecimento e secagem.

Physical properties of dystrophic Red Latosol (Oxisol) under different agricultural uses

Obtaining information about soil properties under different agricultural uses to plan soil management is very important with a view to sustainability in the different agricultural systems. The aim of this study was to evaluate changes in certain indicators of the physical quality of a dystrophic Red Latosol (Oxisol) under different agricultural uses. The study was conducted in an agricultural area located in northern Paraná State. Dystrophic Red Latosol samples were taken from four sites featuring different types of land use typical of the region: pasture of Brachiaria decumbens (P); sugarcane (CN); annual crops under no-tillage (CAPD); and native forest (permanent conservation area) (control (C)). For each land use, 20 completely randomized, disturbed and undisturbed soil samples were collected from the 0-20 cm soil layer, to determine soil texture, volume of water-dispersible clay, soil flocculation (FD), particle density, quantity of organic matter (OM), soil bulk density (Ds), soil macroporosity (Ma) and microporosity (Mi), total soil porosity (TSP), mean geometric diameter of soil aggregates (MGD), and penetration resistance (PR). The results showed differences in OM, FD, MGD, Ds, PR, and Ma between the control (soil under forest) and the areas used for agriculture (P, CN and CAPD). The soils of the lowest physical quality were those used for CN and CAPD, although only the former presented a Ma level very close to that representing unfavorable conditions for plant growth. For the purposes of this study, the physical properties studied were found to perform well as indicators of soil quality.

Sediment, nutrient and water losses by water erosion under agroforestry systems in the semi-arid region in northeastern Brazil

Inadequate soil management practices adopted in the Brazilian semi-arid region contribute to erosive processes. Agroforestry systems (AFs) have been considered an alternative to reduce water erosion. This study aimed to evaluate the impact of two alternatives AFs, a traditional and an intensive cropping system on the losses of sediments, water, organic carbon and nutrients caused by water erosion in comparison to the natural vegetation (caatinga) in a semi-arid region of northeastern Brazil. The agroecosystems studied were: agrosilvopasture (AGP) which consisted of an alley cropping system, cultivated with Leucaena leucocephala and maize, within an area composed by 22% of native trees (200 native trees per hectare) which was grazed during the dry season; silvopasture (SILV) that was composed by 38% of native trees (260 trees per hectare) with a stocking rate of 20 ewes during whole year; traditional agrosilvopasture (TRAG) being managed as following: total deforestation, burning of the residues, cropped with maize for 2 years (1998 and 1999) and fallow during 8–10 years; and intensive cropping (IC) system which was deforested and burned in 1997 followed by cultivation of maize from 1998 to 2002, and thereafter by a fallow period of 8–10 years similar to TRAG. Two areas of native forest (NF1, NF2) known as ‘caatinga’, used as grassland during the dry season and as a source of wood, were selected and used as reference of steady state in the comparative study in relation to the cultivated sites. Sediment and water losses as a result of erosion were collected during two rainy seasons, i.e. 2003 and 2004, and nutrients and organic carbon contents were determined. Soil samples were collected and organic carbon, pH in water, pH in KCl, water dispersible clay (WDC) and hydraulic conductivity (K0) were measured. In 2003, sediment and water losses did not differ significantly among all treatments. However, in 2004, TRAG (0.70 Mg ha−1) and NF1 (1.37 Mg ha−1) showed the highest sediment losses, whereas TRAG and IC presented the highest water losses. On average, nutrients losses in cropped areas were lower than in natural vegetation (NF1, NF2). The alternative AFs (AGP, SILV) were efficient to reduce water erosion effects when compared to the most common agricultural practices adopted in the region, being highly recommended as sustainable technical alternatives for food production in the region.

Alterações físicas e químicas de um Argissolo pela aplicação de água residuária de bovinocultura

Se bem planejada, a disposição de águas residuárias no sistema solo-planta poderá trazer benefícios, tais como fonte de nutrientes e água para as plantas, redução do uso de fertilizantes e de seu potencial poluidor. O solo apresenta grande capacidade de decompor ou inativar materiais potencialmente prejudiciais ao ambiente, através de reações químicas e por processos microbiológicos. Com o objetivo de avaliar os efeitos da aplicação da água residuária de bovinocultura (ARB) sobre as propriedades físicas e químicas de um Argissolo Vermelho Eutrófico, realizou-se um experimento utilizando-se quatro taxas de aplicação da ARB (25, 50, 75 e 100 kg ha-1 de K), em condições de lisímetros de drenagem e em ambiente protegido. Avaliaram-se: condutividade elétrica da solução do solo; argila dispersa em água; índice de saturação por sódio; pH; CTC; saturação por bases e os teores de P, K, Ca, Mg e Na. Os valores de pH, CTC, saturação por bases e concentrações de P, K, Ca e Mg, aumentaram com acréscimos nas taxas de aplicação da ARB, nas camadas superficiais do solo. Não ocorreu salinização do perfil do solo embora a porcentagem de argila dispersa em água tenha aumentado, indicando que a aplicação permanente da ARB pode propiciar riscos de redução da permeabilidade do solo.

Estimation of aggregate stability indices in Mediterranean soils by diffuse reflectance spectroscopy

The proportion of water stable aggregates larger than 0.25 mm (WSA), mean weighted aggregate diameter (MWD) and water dispersible clay (WDC) are three good indicators of the risk of surface sealing, runoff generation and soil erosion by water. These aggregation indices are influenced by the contents in clay, iron oxides, calcium carbonate and organic matter, among other soil properties. Because these soil components possess specific spectral characteristics, one can use reflectance measurements to estimate the previous aggregation indices. To this end, we sampled the surface layer of 80 soils developed on granodiorites, quartzites, sandstones, shales and marls in southwestern Spain. Diffuse reflectance spectra were recorded with an Ultraviolet–Visible–Near Infrared (UV–Vis–NIR) spectrophotometer equipped with a diffuse reflectance sphere for the 300–2500 nm range, a Vis–NIR spectrophotometer with a spinning sample module for the 400–2500 nm range, and a mid infrared (MIR) spectrophotometer with a diffuse reflectance hemisphere accessory for the 4000–500 cm − 1 (2500–20,000 nm) range. Partial least-squares regression analysis based on log (1 / Reflectance) for various spectral ranges provided good estimates of the concentrations of calcium carbonate ( R 2 = 0.95; standard error of validation (SEV) = 56 g kg − 1 ), clay ( R 2 = 0.84; SEV = 54 g kg − 1 ), free iron oxides ( R 2 = 0.83; SEV = 3.3 g kg − 1 ), and acceptable estimates for pH ( R 2 = 0.69; SEV = 0.6), organic matter ( R 2 = 0.41; SEV = 5.2 g kg − 1 ), WSA ( R 2 = 0.60; SEV = 67 g kg − 1 ), MWD ( R 2 = 0.62; SEV = 0.4 mm), and WDC ( R 2 = 0.66; SEV = 29 g kg − 1 ). The instrument/wavelength range combination providing the highest R 2 values and accuracy (lowest SEV) was found to depend on the particular soil property. The goodness of prediction from measurements of the fine (< 2 mm) earth and ground (< 0.1 mm) samples was similar. Overall, our results suggest that diffuse reflectance spectroscopy is useful for (i) categorising soil zones according to aggregate stability (and hence susceptibility to water erosion); and (ii) as a screening tool for deciding whether any further laboratory analyses are needed, among other purposes.

Sorption of High Explosives to Water‐Dispersible Clay: Influence of Organic Carbon, Aluminosilicate Clay, and Extractable Iron

Explosives in soils can present environmental problems for military installations. Fine, mobile particles represent the most reactive fraction of the soil and, therefore, are expected to adsorb explosives and potentially facilitate their transport. The objective of this study was to determine the relative significance of phyllosilicate clay, organic matter, and two forms of extractable iron in adsorption of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by the colloidal water-dispersible clay (WDC) fraction of the soil. The WDC fraction of two mineral and one organic soil was separated and then treated to remove organic carbon (OC) and several forms of iron (Fe(o), oxalate extractable, and Fe(d), dithionite-citrate extractable). Adsorption coefficients were determined for whole soils, untreated, and treated WDC. For mineral soils, adsorption of TNT and RDX on the WDC was greater than on the whole soil. The presence of OC increased explosives sorption by WDC. When OC was removed, iron interfered with TNT sorption. In the presence of OC, removal of Fe(o) decreased RDX adsorption and increased TNT adsorption indicating different adsorption mechanisms. Organic carbon was a more significant indicator of explosives adsorption by WDC than clays or iron oxides and hydroxides. Therefore, OC is the most likely medium for facilitated transport of TNT and RDX.

Settling Velocity, Aggregate Stability, and Interrill Erodibility of Soils Varying in Clay Mineralogy

Mechanisms of aggregate disruption and the measurement techniques used to quantify them for different aggregate sizes affect the relation of aggregate stability to soil erodibility and to basic soil properties. We evaluated two different techniques of aggregate stability analysis that gave either a settling velocity or stability of aggregates parameter for different sized aggregates which we compared with interrill erodibility for 10 clay soils. We compared the differences in these parameters from slow wetting to reduce slaking to air‐dried aggregates and compared these differences to soil properties. Aggregate settling velocity and stability and soil interrill erodibility were strongly affected by clay mineralogy and physical–chemical properties. The mechanism of aggregate disruption was dependent on clay type. Slaking during fast wetting was important in kaolinitic/oxidic soils, whereas highly smectitic clay increased particle dispersion and slaking on swelling, with a consequent reduction in size and speed of settling aggregates. Swelling of clays may have overridden any reduction in slaking by slow capillary prewetting of illite or smectite (with no kaolinite) soils, causing aggregate instability with both slow and fast wetting procedures. Correlation analysis showed that 4.76‐ to 8‐mm aggregates with a high slaking index also demonstrated more slaking under wet sieving and slower fall velocity. Interrill erodibility had greater correlation with the mean weight diameter (MWD) of stable aggregates in the 1‐ to 2‐mm size class, than for the whole soil (aggregates &lt; 8 mm), and no correlation was observed with any of the slaking indexes involving wet sieving or settling in water. Multiple regression analysis indicated that 89% of the variability in erodibility for prewetted soil was explained by MWD of prewetted 1‐ to 2‐mm stable aggregates (MWDW), available water content, and fall velocity of 1‐ to 2‐mm dry aggregates, while 96% of the variability in erodibility for dry soil was explained by MWDW for 1‐ to 2‐mm prewetted aggregates, water dispersible clay, and fall velocity for 1‐ to 2‐mm dry aggregates. The interrill erodibility, for dry and wet soil, was greatest for the highly smectitic and least for the high‐clay kaolinitic/oxidic, both under annual crops. The higher erodibility and lack of slaking reduction effect on our prewetted soil under simulated rainfall is explained by a confounding effect of high water table, high steady‐state runoff and slaking.

Micromorphology of surface crusts in the Knersvlakte, South Africa

Soils in the Knersvlakte are particularly prone to crusting and have lower inherent infiltrability than other soils across western southern Africa. Micromorphological techniques were used to examine the structure and porosity of soil crusts in the Knersvlakte to ascertain why crusting is so intense in this region. Quantile regression using boundary lines was employed to examine the relationships between infiltrability and soil properties for all samples (n = 67). This analysis showed that infiltrability is potentially maximal at low waterdispersible ‘clay plus silt’ content and low silt content (r2 = 0.72 and 0.64; respectively, n = 67) (Figure 2). The strength of crusts, pH, EC, clay mineralogy, and water-dispersible clay, silt and ‘clay plus silt’ content were compared, and a pore analysis using optical microscopy was undertaken on images of six soil thin sections (n = 6) (circular and parallel polarizers). Pore analysis was further undertaken on five horizontal slices of equal dimensions taken through each soil thin section. The porosity samples with low infiltrability ( 100 mm·hr−1, n = 2). The porosity samples with low infiltrability showed a trend of lower pH and greater water dispersible clay percentage. Porosity varied within the porosity samples due to the presence of dense clay/silt bands (< 0.5 mm in width) with relatively few air vesicles. The porosity samples with horizontal slices of low porosity (but large numbers of air vesicles) had low infiltrability, while those without slices of low porosity (and relatively few air vesicles) had high infiltrability. We conclude that the intense crusting and resultant low infiltrability of soils in the Knersvlakte appears to be related to the formation of thin, dense clay/silt bands in the pedoderm.

Colloidal stability in some tropical soils of southeastern Nigeria as affected by iron and aluminium oxides

Abstract The stability of microaggregates in soils as opposed to its dispersion is a very important soil phenomenon that checks degradation arising from unguided tillage and soil erosion. Ten soils from southeastern Nigeria were sampled from their typical A and B horizons for the study. The aim was to identify the extent of colloidal stability of the soils and the forms of Fe and Al oxides in the soils contributing to their stability. The soils are mostly Ultisols and Inceptisols formed on sandstones and shale parent materials. The soils are low in soil basic cations including the soil organic carbon (SOC). The major clay mineral is kaolinite while the soil is acid in reaction. The various forms of soil Fe and Al oxides are high with the total forms of Fe and Al being most dominant and > dithionite extracted Fe and Al > oxalate extracted Fe and Al > pyrophosphate extracted Fe and Al. The water-dispersible clay and silt (WDC) and (WDSi) which are index of dispersion in most soils are low to medium thus reflecting in the low to medium dispersion ratio (DR). The clay flocculation index (CFI) and aggregated silt + clay (ASC) were moderate to high implying the high potential stability of the soils. Soil organic carbon did not seem to be contributing much to the stability of the microaggregates while oxalate and pyrophosphate extractable Fe (Feox, Fep) and to some extent total Al (Alt) were among the different forms of oxides that act as aggregating agents. We propose here that rather than SOC acting as a disaggregating agent in the soils, it might have acted in association with these oxides in a linkage or bridge such as C–P–OM–C to ensure stability of the soils.

Sorption and desorption of atrazine and diuron onto water dispersible soil primary size fractions

In this study, a low energy separation method was employed to separate water dispersible clay-, silt-, and sand-sized fractions. The batch equilibrium method was used to conduct atrazine and diuron sorption/desorption experiments with the bulk soils and their size fractions separately. A Freundlich sorption model provided the best fit for all sorption and desorption data. A mass balance calculation, taking into account the pesticide concentration differences in the size fraction and bulk soil, showed that pesticide sorption onto the different size fractions reproduces well the total amount of the pesticide sorbed onto the bulk soils. Due to their higher soil organic carbon content, the clay fractions were much more effective sorbents for the pesticides than the bulk soils, silt, and sand fractions. For all soils, the amount of the pesticide sorbed onto the clay fractions was more than 20% of the total amount of the pesticide sorbed by the bulk soils even though the clay fractions in these soils were only 5.3–14.0% (by weight). The clay fractions had the highest desorption hysteresis among all size fractions and the bulk soils, followed by silt fractions, implying the clay fractions had the strongest bound and least desorbable pesticide molecules. Our results suggest that attention should be paid to the pesticide sorbed to the smallest colloids, the water dispersible fraction, which can be potentially mobilized under field conditions, leading to wide spreading of contamination.